Darkroom Session: The Latent Image Made Visible

Two hours, one packet of paper and three trays, and you will have measured the thing the last four pages have been describing: how much an invisible cluster of a few silver atoms is worth once a developer finds it. You will also find out whether your latent images survive a week, which colours of light your paper can actually see, and whether the lamp you have been calling a safelight deserves the name.

Purpose

To make the latent image measurable on ordinary silver-gelatin paper, by finding the exposure at which it first becomes developable and comparing that with the exposure at which silver becomes visible without any developer at all; and then to test its keeping, its colour response and the safety of your own darkroom lighting.

Hypothesis. Four statements, each of which the session can support or refuse.

1. The exposure needed to make an image develop is smaller than the exposure needed to make one print out by several orders of magnitude, because development amplifies a few atoms into a whole crystal’s worth of silver.
2. A latent image on paper survives a day with no useful loss, as its maker states, and loses measurably more over a week, with the loss largest in the least-exposed steps.
3. Ordinary black-and-white paper responds to blue and green light and not to red or orange, and a graded blue-sensitive paper responds even less to green than a variable-contrast one does.
4. Your safelight, at your working distance, produces no detectable density change in four minutes on paper that has already had its printing exposure.

The control. Every station has one, and none of the results means anything without it.

• Station 1: an unexposed strip, developed alongside, that fixes the paper’s base density.
• Station 2: two controls — an unexposed strip developed at each session, which measures fog; and a strip exposed and developed at the later session, which measures whether the paper itself has changed. Without the second, a thin week-old strip could equally mean a decayed latent image or a lazier emulsion, and Sheppard and Mees warned about exactly that confusion in 1907.
• Station 3: an unfiltered step on the same sheet, so every gel is read against the light that reached the paper with nothing over it.
• Station 4: the 0-minute step, which is the same sheet with no safelight exposure at all.

The one variable that changes. Station by station: the developer (present or absent) in Station 1; the elapsed time between exposure and development in Station 2; the colour of the light in Station 3; the duration of safelight exposure in Station 4. Everything else — paper, batch, lamp, distance, developer, dilution, temperature, time, agitation — is held constant across each station, and that is why the processing discipline below is not housekeeping but part of the measurement.

Learning objectives

By the end you will be able to:

• Determine a threshold exposure and state it in seconds and in stops.
• Give a defended order-of-magnitude estimate of what development is worth, and say precisely what your measurement does and does not establish.
• Design a keeping test with the two controls it needs, and explain why one is not enough.
• Map the colour response of a paper you own, and connect it to the way its maker describes it.
• Run a safelight fog test to a published method and say whether your room passes.
• Process paper with the consistency that makes strips comparable, and keep a record that lets you compare them next month.
• Make and finish a composed photogram for the portfolio.

Prerequisites

The Latent Image, Reciprocity Failure and the Life of the Latent Image and Spectral Sensitivity and Colour Response, which between them supply everything this session measures; Development as Amplification for what the tray is doing. From Part II: commissioning your laboratory, concentration and dilution, which supplies the dilution arithmetic and is not repeated here, and chemical waste and silver waste.

Safety classification

Level A: a standard home darkroom. The criteria of the rubric that decided it:

• Substances. Everything handled is a dilute working solution of a proprietary photographic chemical at the maker’s own dilution. The paper developer is alkaline — ILFORD publish pH 10.45 to 10.55 for Multigrade at 1+9 — and is described by them as a dimezone-s/hydroquinone developer; hydroquinone’s aggregated GHS classification includes skin sensitisation and serious eye damage. The stop bath is a citric acid solution at 1+19. The fixer is an ammonium thiosulfate rapid fixer at 1+4, pH 5.0 to 5.5. All are irritants at these dilutions, handled in tens of millilitres to a litre, which is Level A territory.
• Energies. One mains lamp and, if you have one, an enlarger — both purchased, certified equipment, neither modified. Nothing is heated: the solutions sit at room temperature, nominally 20 °C.
• Procedures. Trays of dilute solution at room temperature, moved with tongs. The failure mode is a spoiled strip, not a burn.
• Waste. Used fixer is silver-bearing and goes to the silver waste stream, which Part II established.

What is not a hazard here, and why. The silver in this session spends almost all of it locked in a hardened gelatin layer on a sheet of paper. Dry paper is inert to handle: there is no dust, because nothing is powdered; there is no soluble silver, because the silver halide is essentially insoluble and bound in the coating; and there is no vapour, because nothing evaporates from a sheet of paper. Silver becomes mobile only in the fixer, where thiosulfate complexes it into solution — and that is exactly why the used fixer is the one liquid on this page that must not go to the drain. Contrast this with the halide experiment earlier in this part, where silver nitrate is handled as a solution and the classification rises to Level B: same element, different physical state, different level. That comparison is the assessment, not an afterthought to it.

Hazards

Paper developer: skin sensitisation and eye damage. Hydroquinone’s aggregated GHS classification carries skin sensitisation and serious eye damage, and the working solution is alkaline at around pH 10.5. Sensitisation is the hazard that gets people, because it is cumulative and irreversible: repeated small contacts over months can produce a dermatitis that ends darkroom work. Use tongs, not fingers, which is ILFORD’s own instruction, and gloves as well.

Stop bath and fixer. Citric acid at 1+19 and the fixer at 1+4 are mild irritants that go in an eye as readily as anything else. The fixer’s real problem is what it contains once used, which is dissolved silver.

Cross-contamination. ILFORD’s beginners’ sheet warns that a trace of fixer or stop bath in the developer leads to inconsistent results or, at worst, completely blank prints. Never return a solution to the wrong bottle, never share tongs, and never mix developer and fixer.

The lamp is hot. Stations 1 and 3 hold paper, and in Station 3 plastic gels, near a lamp for long periods, and an incandescent bulb runs hot enough to scorch paper and melt or ignite a gel in contact with it. Keep everything a hand’s width clear, prefer an LED lamp, and never leave a lit lamp unattended with material against it.

Working in the dark. Trip hazards, a wet floor and a tray of liquid at elbow height are the real risks. Lay the room out before the lights go off, and do not change it after.

Required PPE

• Nitrile gloves whenever your hands are near a solution. HSE’s COSHH essentials sheet for manual film and plate development takes single-use nitrile as splash protection where the safety data sheet gives nothing more specific, and that is the basis used here.
• Eye protection: safety glasses at minimum, worn from before the first bottle is opened. ILFORD recommend gloves, eye protection and an apron or overall for handling and mixing all their chemicals.
• An apron or overall, and clothes you do not mind marking. Developer stains.
• Print tongs, one pair per tray, marked and never swapped. Tongs are PPE here and not a convenience: they are the control that keeps a sensitiser off your skin for the fifty or so immersions this session involves.

Ventilation

Ventilation is a control here, and it is the ordinary one: work in a room with an openable window or an extractor, and do not process in a sealed cupboard. ILFORD’s own guidance is to ensure darkroom areas are well ventilated. Nothing here produces a vapour that needs extraction — the solutions are cold, dilute and aqueous, the stop is citric rather than acetic and its maker describes it as low odour, and no gas is generated — so a fume cupboard is not among the controls. Air movement is here for the general reason: a small dark room with three trays gets stuffy, and a person working carefully in a stuffy room stops working carefully.

Materials

Item	Quantity	Notes
Variable-contrast RC paper, 5 × 7 in or cut strips	about 20 sheets	ILFORD Multigrade RC or equivalent. Buy one box; this session and Part V will use it.
Graded paper, any grade	4 sheets	For Station 3. ILFORD Ilfobrom Galerie FB or a Foma graded paper.
Coloured gels: red, orange, green, blue	1 swatch each	Theatrical lighting gel; sample swatch books are often free. Record the maker and the number of each.
Opaque card	2 pieces	For masking exposure steps.
Kitchen aluminium foil	a sheet	Light-tight wrapping for the strips that wait a week.
Light-tight envelope or paper safe	1	Anything a box of paper came in will do.
Objects for the photogram	a handful	Leaves, lace, glass, keys, cut paper: choose for a range of transparency and colour.
Timer with a second hand	1	A phone with the screen dimmed and turned face down between readings.
Pencil and a printed record sheet	—	Pencil, because ink runs.

Chemicals

Chemical	Quantity	Form
Paper developer concentrate	100 mL	Diluted 1+9 to make 1 L of working solution. ILFORD describe MULTIGRADE as a dimezone-s and hydroquinone developer.
Stop bath concentrate	50 mL	Diluted 1+19 to make 1 L. ILFOSTOP is a low-odour citric acid stop bath; a dilute acetic acid stop is the alternative.
Rapid fixer concentrate	200 mL	Diluted 1+4 to make 1 L. ILFORD Rapid Fixer is an ammonium thiosulfate fixer and contains no sodium thiosulfate.
Water	about 10 L	For dilution and washing.

The dilution arithmetic is Part II’s and is used here rather than restated: 1+9 means one part concentrate to nine parts water, so 100 mL of concentrate makes 1 L of working solution. Note that this course writes dilutions as 1+9 and never as 1:9, because different sources use the second notation to mean different things.

📓What to write down before you start

Record the make, product name and batch or expiry of the paper; the developer, its dilution and its temperature; the stop and fixer dilutions; the lamp (type, rated power) and its distance from the paper; the safelight (filter, bulb wattage, distance) and the age of its filter. Half of these will feel pointless today and all of them will matter when you repeat a station in six months and get a different answer.

Equipment

Three trays large enough for your paper and a fourth for washing, or a sink; a 1 L measuring cylinder and a dedicated mixing jug; a thermometer reading to 0.5 °C; three pairs of print tongs; a safelight; a lamp on a stand or clamp that can be fixed at a measured distance; a tape measure; clean glass or a contact frame heavy enough to hold objects flat; a table you can black out; and, if you have one, an enlarger, which makes Stations 1, 2 and 4 far easier to control.

Estimated cost

Cost band £, and the cheapest practical session so far: about twenty sheets of resin-coated paper, four of graded paper, and a few hundred millilitres of three concentrates that will each do several more sessions. Everything else is equipment the rest of the course reuses. Dated prices live in the laboratory planner rather than in this text.

Estimated consumables cost

Four stations, twenty-four sheets of paper and three concentrates. The concentrates are the interesting case: 350 mL of concentrate makes three litres of working solution, which is far more than this session needs, so the row below prices what is poured, not what is used up.

Consumed	This session	Sourced price	Cost this session
Variable-contrast RC paper, 5 × 7 in	about 20 sheets	£16.06–£44.71 per 25 to 100 sheets, 5 x 7 in, variable contrast RC (£0.45–£0.64 a sheet)	£8.94–£12.85
Graded paper, any grade	4 sheets	None. graded-paper carries a cost band and no dated figure	—
Paper developer concentrate	100 mL, diluted 1+9 to make 1 L	£10.52–£20.03 per 500 ml to 1 L of concentrate, diluted 1+9	£2.00–£2.10
Stop bath concentrate	50 mL, diluted 1+19 to make 1 L	£10.66–£12.18 per 500 ml of citric acid concentrate, diluted 1+19	£1.07–£1.22
Rapid fixer concentrate	200 mL, diluted 1+4 to make 1 L	£21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film	£4.21–£5.20
Coloured gels: red, orange, green, blue	one swatch of each	Swatch books are often free	—
Water	about 10 L	Metered supply; the planner prices no water	—
Kitchen foil and a light-tight envelope	a sheet; the envelope is reused	None. box-and-foil carries a cost band and no dated figure	—

The priced rows come to £16.22 to £21.37 for one run of this session, at the retail ranges read on 5 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 2 of the 8 rows carry no dated price, so they are counted as nothing here and are certainly not free. A priced entry is a dated range to plan against, never a quotation.

Trays, tongs, graduates, thermometer, safelight, lamp and contact glass are equipment and belong to the band above. Graded paper is the one photographic consumable here with no dated price.

Waste streams

Used fixer is silver-bearing and is the one stream that must be collected. Every millilitre of it, plus the first wash water off the prints, goes into the labelled silver waste container Part II established. Used developer and used stop are not silver-bearing. Scrap paper, once fixed and washed, is ordinary household waste.

ILFORD’s guidance for UK domestic users is that local authorities usually accept small quantities of chemical waste at Household Waste and Recycling Centres, bottled separately and labelled, and that different wastes should never be mixed for disposal. Local regulation governs and the arrangements differ between authorities and countries; find out what yours are before the session, not after.

Alternative route

This page needs a room that can be made dark and a mains lamp. If you have neither yet:

Without a darkroom. A windowless bathroom at night, with a towel along the bottom of the door, is the accepted route and is what Part IV’s overview assumes. A changing bag will let you load and unload paper but is too small to process in.

Without a safelight. Work in total darkness. Every station except Station 4 works in the dark, and Station 4 is the one test you cannot run without a lamp, so it waits until you have one — the-safelight-fog-test in Part XVI.

Without a controllable lamp. Station 1’s develop-out arm runs with a torch at a measured distance, or with daylight through a small aperture. All that matters is that the source is steady and that its distance is recorded.

Preparation

1. Read Stations 1 and 2 in full before the session, because Station 2 is exposed on day one and finished on day eight, and a strip you forget to expose today cannot be rescued.
2. Lay the room out with the lights on. Trays left to right: developer, stop, fixer, wash. A dry side and a wet side, and never a wet hand on the dry side.
3. Mix the solutions to the maker’s dilutions and bring them to 20 °C ± 1 °C, checking with the thermometer rather than by feel. Fill each tray about half deep.
4. Cut your paper. Under the safelight, cut about 16 strips of roughly 25 × 130 mm from the variable-contrast paper and 4 from the graded, and return them to the light-tight envelope. Notch one corner of every strip so you can find the emulsion side in the dark.
5. Fix the lamp. Clamp it at a measured height above the working surface and do not move it again. Write the distance in the record.
6. Range-find the exposure. Give one strip a doubling series under the lamp from a fraction of a second upwards, develop it, and see where a just-perceptible tone first appears. If it appears on the first step, move the lamp back or stop the lens down and repeat; if nothing shows, come closer. This is not a waste of paper: both Kodak and ILFORD begin their published safelight tests with the same ranging exposure, and without it your series lands off the end of the scale.

Procedure

Work through the stations in order. Stations 1, 3, 4 and 5 are finished today; Station 2 spans a week.

Station 1 — Amplification: what is the developer worth?

Arm A, the develop-out threshold. Under the safelight, lay a fresh strip on the working surface at the fixed lamp distance and give it a doubling series with the card as a moving mask, using the times you settled on during ranging — 1, 2, 4, 8, 16, 32, 64 units, one stop per step. Leave one end unexposed. Process it (see Processing discipline below). The develop-out threshold is the shortest exposure whose tone you can just distinguish from the unexposed end.

Arm B, the print-out threshold. Expose a second strip, without any development afterwards, to as much light as you can give it in the time available: bring the same lamp as close as is safe and record the new distance, or, better if the weather allows, put the strip behind glass in direct sunlight. Start it at the beginning of the session and leave it to the end, with an identical strip wrapped in foil beside it as the shielded control. At the end, compare the two by eye, then fix both for 30 seconds, wash and dry them so the comparison survives. Whether you see anything at all is itself the result, and the Analysis handles both outcomes.

Station 2 — Keeping and fog

Expose five strips today, all identically, with three steps only: the develop-out threshold, threshold + 2 stops and threshold + 4 stops. Label them and treat them as follows.

Strip	Exposed	Developed	What it measures
2A	today	today	the reference
2B	today	tomorrow	keeping over 24 hours, which the maker states is safe
2C	today	in a week	keeping over a week, which nobody states
2D	in a week	in a week	whether the paper itself has changed
2E	not exposed	today	fog, today
2F	not exposed	in a week	fog, in a week

Wrap 2C, 2D and 2F separately in foil, then in the light-tight envelope, and keep them somewhere cool, dry and dark — the conditions every manufacturer specifies for exposed material. Record where. Process 2A and 2E today, 2B tomorrow, and the rest on day eight, using the same developer dilution, temperature and time.

Station 3 — The spectral map

Take one strip of variable-contrast paper and one of graded. Lay the four gels side by side across each, leaving a band at one end uncovered as the control, and weight them flat under the glass. Give both the same exposure under the same lamp, about four stops above the develop-out threshold, so the uncovered band reaches full black and the gels have something to subtract from. Process both together, in the same tray, at the same time, so that no difference between them can be a processing difference.

Station 4 — The safelight test

This follows the published method, and the order of the steps is what makes it work.

1. With every light off, give a strip the enlarger or lamp exposure that produces a light grey tone — Kodak specify a reflection density of 0.25 to 0.50, which is about two stops above your develop-out threshold. Label this end “after”.
2. Carry it, still in the dark, to the place in the room where the safelight illumination is brightest — usually over the developer tray.
3. Lay it emulsion-up on a card and, using an opaque card as a mask, give successive areas 0, 1, 2 and 4 minutes of safelight exposure. ILFORD’s method uses those four steps; Kodak’s uses 0, 1, 3 and 7.
4. In the dark again, give the other end of the strip an identical grey-tone exposure, and label it “before”.
5. Process the strip in total darkness, alongside an unexposed strip that has had no safelight exposure at all.

The point of steps 1 and 4 is the one most home-made safelight tests miss. Kodak state it plainly: a test that merely covers part of a sheet with a coin and exposes it to the safelight checks only for fog, not for the added effects of safelight and enlarger exposure together. Paper is more sensitive to safelight fogging after it has been exposed than before, which is the latensification effect of the reciprocity page, so the fog-only test can pass a lamp that is quietly ruining your highlights.

Station 5 — The photogram

Now make something. Arrange objects on a fresh sheet, emulsion up, under the glass — an opaque key, a translucent leaf, textured glass, a coloured gel offcut — and compose deliberately rather than scattering. Expose at about threshold + 3 stops and process fully. This sheet is for the portfolio, so wash it properly and dry it flat.

🛠Processing discipline — the same for every strip in the session

Times and temperatures from the maker’s own processing summary for resin-coated paper, at 20 °C:

Step	Solution	Dilution	Time
Develop	paper developer	1+9	1 min
Stop	citric acid stop	1+19	10 s
Fix	rapid fixer	1+4	30 s
Wash	running water	—	2 min

Agitation: rock the tray gently and continuously from the moment the strip goes in. Drain: lift the strip out ten seconds before the development time ends and let it drain before it enters the stop, which is the maker’s own instruction and why the nominal time is a minute and not seventy seconds.

The rule that makes the session a measurement. Every strip to be compared with another gets the same dilution, the same temperature within a degree, the same time within two seconds and the same agitation. If you cannot hold 20 °C, hold whatever you have and record the number: a steady 18 °C is a usable experiment, a drift from 22 °C to 17 °C is not.

Between sessions, mix fresh developer; ILFORD state that high temperatures reduce solution life considerably, and a working-strength paper developer does not keep. Fixer at 1+4 will do a hundred 8 × 10 prints per litre and keeps in a labelled bottle until exhausted.

The station plan: what is fixed, and what moves

1. 1Lamp at a measured height — the one dimension held constant all session; write it down
2. 2Marked working area — every strip is exposed on this rectangle, not near it
3. 3Dry side — paper, cards, record sheet, pencil
4. 4Wet side: developer, stop, fixer, wash — one pair of tongs per tray, never swapped
5. 5Safelight, not less than 1.2 m away — Station 4 tests this exact geometry
6. 6Silver waste container — used fixer and the first wash water

A plan, not a scale drawing. The only measurement that matters here is the lamp distance, and its value belongs in your record rather than on this diagram, because it depends on your lamp.

Expected observations

Station 1, Arm A. A clean staircase deepening by one step per stop, from paper white at the unexposed end to full black at the longest exposure. The threshold step is often hard to call, which is worth noticing: your eye is the instrument and a just-perceptible tone is a judgement. Read it against the unexposed end, in even light, with the strip dry.

Station 1, Arm B. Most likely nothing at all, or the faintest greying seen only by tilting the strip against the shielded control; in strong direct sun, a very faint violet-brown tone. Either outcome is a result.

Station 2. 2A and 2B indistinguishable, since the maker states no significant change over 24 hours. 2C possibly a little thinner, and if the loss is real it should show most in the threshold step and least in the +4 step, which is Baekeland’s observation that under-exposed material loses more. 2D should match 2A; 2E and 2F should be paper-white or nearly so.

Station 3. On the variable-contrast strip, the blue and green panels dark, the orange and red panels near paper white, the uncovered control darkest of all. On the graded strip, the blue panel dark, the green panel noticeably lighter than on the variable-contrast strip, and red and orange white.

Station 4. A room that passes shows no difference between the 0- and 4-minute areas at either end. A room that fails shows it first on the “after” end, and only at longer times on the “before” end.

Station 5. Sharp black-edged shadows where objects touched the paper, softening where they did not, and a range of greys through the translucent objects.

What is happening chemically

In the exposure. A photon above the paper’s threshold is absorbed in a microcrystal, freeing an electron and leaving a hole. The electron is trapped at a sensitivity centre, an interstitial silver ion arrives and is reduced to a silver atom, and the steps alternate until a cluster of about four atoms exists. Nothing visible has happened, and that is the entire content of Arm A’s exposure.

Ag⁺ + e⁻ → Ag

Repeated about four times per crystal: the whole of the latent image

In Arm B, with no developer, that reaction has to be driven to a visible quantity of silver by light alone, and two things make it expensive: it takes tens of thousands of atoms per particle rather than four, and the liberated halogen must go somewhere. Ware notes that at print-out exposures gelatin is not an effective halogen scavenger, so on a developing-out paper much of the photolytic silver is re-oxidised as fast as it forms. That is why the ratio you are measuring is large.

In the developer. The cluster acts as an electrode: the developing agents give up electrons to it from solution, the electrons conduct through the metal to the silver–halide boundary, silver ions are reduced there and bromide is released. The crystal is reduced entire.

AgBr + e⁻ → Ag + Br⁻

Chemical development, per formula unit

In the stop bath. Development needs an alkaline environment; the citric acid stop drops the pH and halts it within seconds, which is why ten seconds is enough, and it protects the fixer from carried-over alkali.

In the fixer. Thiosulfate turns the remaining silver halide into soluble complexes and carries it out of the paper — the complex-formation chemistry of Part III. This is where the silver that is not image leaves, and why used fixer is the silver-bearing stream.

In the wash. Diffusion, as Part III taught: the complexes and the excess thiosulfate leave the coating down their concentration gradients, and a resin-coated paper washes quickly because its base does not absorb.

Data to record

For every strip: its label, the paper and batch, the exposure steps in seconds, the lamp distance, the dates and times of exposure and development, the developer temperature, and where it was stored in between. Then, for each station:

• Station 1: the threshold step in seconds for Arm A; the lamp distance for each arm; the total exposure time for Arm B, and whether any tone appeared; the weather and time of day if you used sun.
• Station 2: for each of the six strips, a description of all three steps against 2A, plus a number if you have a densitometer or a scanner you can calibrate.
• Station 3: each gel’s maker and number, and which of the two papers it left white, grey or black.
• Station 4: the safelight filter, bulb wattage, distance and filter age, and the shortest safelight time at which any difference is visible on the “before” and on the “after” ends.
• Station 5: the exposure, and one line on what you would change compositionally.

The result sheet: one page that holds the whole session

1. 1Station 1: the exposure series and both thresholds — ring the threshold step; record both lamp distances
2. 2Station 2: six strips against three steps — plus the dates and the storage conditions
3. 3Station 3: four gels against two papers — and each gel’s maker and number
4. 4Station 4: before and after, at 0, 1, 2 and 4 minutes — then the safe time, which is half the first visible change
5. 5Standing conditions, filled in once — paper and batch, developer, dilution, temperature, times, safelight, date

Rule it up before the lights go off. A result you have to reconstruct from memory afterwards is not a result, and the standing conditions along the foot are what make today's sheet comparable with the one you fill in next year.

Analysis

This is the part that turns strips into the chain the whole part has been building. Work through it with the strips in front of you.

1. The amplification ratio, and what it means

If Arm B produced a visible tone, you have two thresholds and can take their ratio. The arms were at different lamp distances, so convert with the inverse-square law: a lamp at 0.25 m gives (2.0/0.25)² = 64 times the illuminance it gives at 2.0 m.

R = (d_A / d_B)² × (t_B / t_A)

The measured threshold ratio

where d_A and t_A are the distance and threshold time for the develop-out arm and d_B, t_B the same for the print-out arm. Express it in stops as well, because that is the unit a photographer thinks in:

n = log₂ R = log R / log 2

The same ratio, in stops

A ratio of 10⁵ is about 17 stops. A ratio of 10⁶ is about 20 stops.

If Arm B produced nothing, you still have a result, and it is a lower bound: the ratio is at least the exposure you gave Arm B divided by the develop-out threshold. Write it as an inequality — “R > 3 × 10⁵, or more than 18 stops” — and state the exposure you gave. A lower bound honestly reported is a measurement; a number invented to fill the gap is not.

Now say what you have measured and what you have not. Three limitations, all from this part’s own lessons:

• It is a ratio of threshold exposures, not the amplification factor of development, which is atoms of developed silver per atom of latent image and which the development page puts at order 10⁷ to 10⁸. Yours is smaller because the print-out threshold is a visible one, not a four-atom one.
• The arms sit at very different points on the reciprocity curve, and reciprocity failure inflates the print-out arm’s requirement, so the ratio over-states what matched conditions would give.
• Ware’s sourced figure for latent against visible photolytic image is of the order of a million, on a print-out material with silver ion and water as halogen acceptors. Your paper has only gelatin, which Ware says is not an effective scavenger at these exposures, so expect to need more than his million.

Write one sentence on whether your result is consistent with that, and why the comparison is not exact.

2. The keeping loss

Lay 2A, 2B, 2C and 2D side by side. Answer three questions in writing.

Is 2C thinner than 2A? If not, you have shown that a week’s keeping produced no loss you can see, which is a real and useful finding — Bothamley reported plates giving unchanged negatives after four years.

If it is thinner, is 2D thinner too? If 2D matches 2A, the loss is in the latent image and you have measured latent-image regression. If 2D is thinner as well, part or all of the loss is in the paper itself, and Sheppard and Mees’s warning has just been demonstrated in your own darkroom.

Where is the loss largest? Compare the threshold step with the +4 step. Baekeland reported that regression is worse in under-exposed than over-exposed material, which is exactly what a threshold argument predicts: a crystal with a bare-minimum cluster only has to lose one atom to fall below developability, while a heavily exposed one has atoms to spare.

Finally, read 2E against 2F. Any density difference between them is fog that accumulated in unexposed paper over the week, which is a different thing again — the paper ageing rather than the image decaying.

3. The fog level

Your fog control, 2E, is the paper’s base plus fog under today’s conditions, and everything else is read against it. Record a number if you can measure one, otherwise describe it against the unexposed border of a fresh sheet. Two things raise it, both under your control: development beyond the recommended time, and developer temperature. Check the temperature first.

4. The gel map

Tabulate which gels blocked the image on each paper, and then do the interpretation, which is the point of the station.

A gel that left the paper white transmitted only wavelengths the paper cannot use. A gel that let the paper go black transmitted wavelengths the paper is sensitive to. That is a two-line spectral sensitivity measurement, made with a lamp and four pieces of plastic, and it is the same measurement a manufacturer makes properly with a spectrograph and a grey wedge.

The comparison between the two papers is the more interesting half. Variable-contrast paper is, in ILFORD’s own description, three blended emulsions each carrying a different amount of green sensitising dye; a graded paper has no such dye. So the green panel should be markedly darker on the variable-contrast strip than on the graded one, and if it is, you have detected dye sensitisation directly. If it is not, ask whether your green gel passes some blue as well — which is why you recorded the gel’s maker and number. This course does not supply transmission curves for gels; look up the published curve for the exact gel you used before you draw a firm conclusion, because a “green” gel that leaks blue will produce a false positive.

5. The safelight verdict

State a verdict, not an impression. ILFORD’s criterion: if there is no density change between the 0-minute and the 4-minute areas, the safelight conditions are safe; if there is a small density change, of the order of 0.04 in density, after just one minute, the conditions are inadequate. Kodak’s definition of safe time is any time less than or equal to half the time at which a detectable change first appears.

So: find the shortest time at which you can see a change on the “after” end; halve it; and that is your working safe time. Compare it with how long a sheet of paper is actually exposed to your safelight during a normal printing session — from taking it out of the box to putting it in the fixer. If your safe time is shorter than your working time, the room fails, and the remedies in order of cost are: move the lamp further away, fit a lower-wattage bulb, replace an old filter, use only indirect light, or develop with the safelight switched off.

Record the verdict, the geometry it applies to, and the date. It is not a permanent property of your room: change the bulb, move the lamp or let the filter age, and it must be run again.

6. Close the loop

Finish the report with one paragraph answering this: which of the four hypotheses did your evidence support, which did it fail to test, and what would you change to test that one properly? That paragraph is the whole point of the session, and it is the one thing a reader of your notebook in five years will still find useful.

Troubleshooting

Every strip came out black. A white-light leak, or paper exposed before you started. Check the door and the enlarger head with eyes dark-adapted for a full fifteen minutes, which is how long adaption takes.

Every strip came out white, including the long exposures. Contaminated developer — ILFORD warn that a trace of fixer or stop in it can give completely blank prints — or the paper went in emulsion-side down. Mix fresh developer into a clean tray and repeat one strip.

Uneven or mottled tones. Insufficient agitation, or the strip floated. Push it under at once and rock continuously.

A brown or green tinge when dry. Under-fixed or under-washed. Fix the full time in fresh fixer, and wash the full time.

The threshold step is impossible to judge. The series is landing in the wrong place: re-range, or use half-stop steps around where you think the threshold is.

Station 3’s gels all look the same. Either the exposure was long enough that even weakly transmitted light passed the threshold, or the gels are less selective than their names suggest. Hold them to a window: if you can read coloured print through all four, they are pale.

The week-old strips are all thin, including 2D. The paper has aged, or the storage was warm or damp. That is a finding, not a failure; record the storage conditions with it.

Clean-up

Pour used developer and used stop into their own labelled waste bottles. Pour used fixer, and the first change of wash water, into the silver waste container. Rinse trays, cylinders and tongs and stand them to dry; wash your hands before touching anything dry; wipe the bench. Note the volume of each solution used: that is the beginning of a capacity record Part XI will make you keep properly.

Storage

Working-strength paper developer does not keep: mix it for the session and discard it after. Stop and fixer keep in filled, capped, labelled bottles; write the date mixed and the sheet count on the label. Unexposed paper goes back into its light-tight envelope and its box, cool and dry; ILFORD state their papers keep in excellent condition for up to two years stored that way. Exposed, unprocessed paper — the Station 2 strips — is stored cool, dry and dark, which is what every manufacturer specifies and what Baekeland’s list of variables explains. Processed prints are washed fully, dried flat and kept out of strong light; the permanence and washing standards themselves belong to the fixing and washing parts, and nothing made today is archival until it has met them.

Disposal considerations

Three streams and three different arguments.

Used fixer is silver-bearing. Silver is very toxic to aquatic life — that is what the H400 and H410 statements on the silver compounds in this part mean — and it is also worth recovering: Kodak’s own silver-recovery literature describes metallic replacement cartridges as the practical route for small volumes. It is collected, not poured away.

Used developer contains an oxidised developing agent and an alkali. It is not silver-bearing but it is not innocuous either, and it should not be mixed with the fixer, both because mixing wastes the silver recovery and because ILFORD’s own guidance is that different waste chemicals should not be mixed.

Used stop is a dilute citric acid solution.

ILFORD’s guidance for UK domestic users is that local authorities usually accept small quantities of chemical waste at Household Waste and Recycling Centres, bottled separately and labelled. Local regulation governs, and it differs between authorities and between countries: find out what applies where you are, and keep the answer in the front of your notebook.

Questions

1. Your develop-out threshold was 2 seconds at 2.0 m and you gave the print-out arm 90 minutes at 0.25 m, with no visible result. What lower bound does that put on the threshold ratio, in stops?
2. Strip 2C is thinner than 2A at every step, and 2D matches 2A. What have you shown, and what have you ruled out?
3. Strip 2C is thinner than 2A only at the threshold step, and matches it at +4 stops. Why is that the pattern a four-atom threshold predicts?
4. Your green gel darkened the variable-contrast paper strongly and the graded paper only slightly. Name the mechanism and the year it was discovered.
5. Your safelight test shows the first visible change at 2 minutes on the “after” end and at 4 minutes on the “before” end. What is your safe time, and which end governs it?
6. You ran Station 3 with a tungsten lamp and a friend ran it with a daylight LED panel. Whose blue panel went darker, and why does the answer make a wedge spectrogram’s stated illuminant important?

Further experiments

Extend the keeping test. Repeat Station 2 at a month and three months, and with one strip kept deliberately warm against one in a refrigerator. That turns Baekeland’s list of variables into a measurement.

Add a fifth gel. A deep red and a very deep red together will tell you whether your paper’s response stops or merely fades, which is Kodak’s point that colour sensitivity does not end abruptly.

Test a second paper. A warm-tone or a Foma paper against the one you used: makers sensitise differently, and the gel map shows it.

Move the safelight and test again. Double the distance and run Station 4 once more. The illuminance falls by four, so you can predict the result before you develop — the most satisfying way to use an inverse-square law.

Compare the photogram with the Part I cyanotype. Put today’s print beside the cyanotype photogram and write half a page on what changed — tonal range, edge quality, colour, exposure time — and which differences come from the chemistry rather than from your having got better at it.

Check your understanding

Question 1. Why does Station 2 include a strip that is exposed and developed a week from now, when the point of the station is latent-image keeping?

• To use up the rest of the paper
• Because it is the control that separates decay of the latent image from a change in the paper itself, which would look identical on a single strip
• Because the developer will be a week older
• To check the safelight a second time

Show the answer and why

Answer: Because it is the control that separates decay of the latent image from a change in the paper itself, which would look identical on a single strip

Sheppard and Mees raised this objection in 1907 and it has not been answered since: photographers confuse the developing properties of a material with the survival of the latent image. They had measured plates whose velocity coefficient of development fell to a third of its former value over six months of poor storage, which thins a negative without any latent-image decay at all. A strip exposed and developed at the later session shares the paper is ageing but not the waiting latent image, so comparing it with the reference isolates one variable from the other. It costs one sheet of paper and it is the difference between an experiment and an anecdote.

Question 2. Station 1 Arm B gives no visible tone after 90 minutes. What should the report say?

• That the experiment failed and should be repeated
• That paper cannot print out
• That the threshold ratio is at least the exposure given divided by the develop-out threshold, stated as an inequality together with the exposure that was actually given
• That the amplification factor is 10 to the seventh

Show the answer and why

Answer: That the threshold ratio is at least the exposure given divided by the develop-out threshold, stated as an inequality together with the exposure that was actually given

A lower bound is a measurement. If the develop-out threshold was 2 seconds and Arm B received 5400 seconds at 64 times the illuminance, the print-out exposure was at least 64 times 5400 divided by 2, or about 1.7 times ten to the fifth times the develop-out threshold, which is over 17 stops, and no tone appeared, so the true ratio is larger still. Reporting that honestly is worth more than reporting a number you did not measure. The fourth option is the specific error to avoid: the order of ten to the seventh figure comes from an arithmetic estimate using published coating weights and grain sizes, not from this strip, and quoting it as a result of the session would be passing off somebody else's calculation as your measurement.

Question 3. Why does the safelight test give the paper an enlarger exposure at both ends of the strip rather than simply timing the safelight against a covered area?

• To make the fog easier to see against a grey background
• Because paper is more sensitive to safelight fogging after it has been exposed, so a fog-only test can pass a lamp that is degrading highlights and contrast in real prints
• Because the enlarger warms the paper
• Because the safelight is brighter near the enlarger

Show the answer and why

Answer: Because paper is more sensitive to safelight fogging after it has been exposed, so a fog-only test can pass a lamp that is degrading highlights and contrast in real prints

Kodak warn explicitly that a test that covers part of a sheet with an opaque object and exposes it to the safelight checks only for fog, not for the added effects of safelight and enlarger exposure together, and can therefore be misleading. ILFORD call the strip given a safelight exposure after the enlarger exposure a check for latensification and the one given it before a check for hypersensitisation, and state that the after case is the more critical. The physics is on the reciprocity page: a sub-developable cluster left by the printing exposure needs only a little more light to cross the threshold, and the safelight supplies it. What you lose is highlights and contrast, in the image area, where you are least likely to notice it.

Question 4. On the Station 3 strips, the green panel is much darker on the variable-contrast paper than on the graded paper. What have you detected?

• That the graded paper is slower
• That the green gel leaked blue light
• Dye sensitisation: the variable-contrast paper carries green sensitising dyes and the graded paper does not
• That the graded paper was fogged

Show the answer and why

Answer: Dye sensitisation: the variable-contrast paper carries green sensitising dyes and the graded paper does not

ILFORD describe Multigrade as a mixture of three emulsions, each a basic blue-sensitive emulsion with a different amount of green sensitising dye added, and the whole contrast-control system depends on that green response. A graded bromide paper has no such dye and is, in their words about chloro-bromide emulsions generally, blue sensitive with a slight sensitivity to green. So the difference between the two green panels is the dye, and you have reproduced Vogel's 1873 observation with two sheets of paper. The second option is the reason the procedure asks you to record the gel's maker and number: a green gel that passes some blue would darken both papers and produce a false positive, so check the published transmission curve before you commit to the conclusion.

Question 5. Your develop-out threshold turns out to fall on the very first step of your exposure series, so you cannot read it. What is the correct response?

• Report the threshold as equal to the first step
• Develop for longer so that the later steps separate
• Move the lamp further away or stop the lens down, and repeat the ranging exposure until the threshold falls in the middle of the series
• Use a faster paper

Show the answer and why

Answer: Move the lamp further away or stop the lens down, and repeat the ranging exposure until the threshold falls in the middle of the series

A threshold that lands on the end of your scale is not measured, only bounded, and the fix is to change the range rather than the reading. Both published safelight test procedures start with exactly this ranging step for exactly this reason, and Kodak note that you will probably have to set the enlarger lens to its smallest aperture and use a very short exposure, because paper is far more sensitive than it feels when you are used to film. Developing longer is the wrong lever: it would raise fog and shift every step together without improving the separation you need at the bottom of the scale.

Question 6. Which single liquid produced in this session must not go to the drain, and why?

• The used developer, because it is alkaline
• The used stop bath, because it is acidic
• The used fixer and the first wash water, because thiosulfate has dissolved silver out of the paper and silver compounds are very toxic to aquatic life
• All of them equally

Show the answer and why

Answer: The used fixer and the first wash water, because thiosulfate has dissolved silver out of the paper and silver compounds are very toxic to aquatic life

Fixing is the step where silver stops being an insoluble solid locked in gelatin and becomes a soluble thiosulfate complex, so the fixer, and the first change of wash water that comes off the print, carry essentially all the silver that did not become image. The aquatic hazard statements on the silver compounds in this part are the reason it is collected rather than poured away, and Kodak's silver-recovery literature describes metallic replacement cartridges as the practical route for small volumes. Developer and stop are not silver-bearing, which does not make them drain-ready either: local regulation governs, ILFORD advise bottling different wastes separately and taking them to a household waste centre, and mixing them would waste the silver recovery as well.

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